1. Field of the Invention
The present invention relates to an error diffusion processing circuit for an image signal. More particularly, the present invention relates to an error diffusion processing circuit for an image signal which improves an output image.
2. Description of the Related Art
For a digitized display, such as a PDP (Plasma Display Panel), gray scale is represented with quantized value, and there is a limit on the number of gray tones. Thus, an image processing method, such as an error diffusing process for an input image, is generally used for apparently increasing the number of the gray tones.
This error diffusion processing method can increase the number of apparent gray tones by diffusing the quantized error into peripheral pixels. The quantized error is defined as a difference between input value and quantized one.
The above-mentioned error diffusion processing method is carried out by an error diffusion processing circuit for the image signal. The error diffusion processing circuit is constituted by a feedback loop circuit. When the image signal such as a high vision signal is targeted, this circuit must be operated at a very high speed. For this reason, a bit precision in the feed back loop needs to be reduced to some degree in some cases. The wider the bit width of the signal, the tighter a calculation speed of an adder and a multiplier in a filter unit of the error diffusion.
FIG. 1 shows a block diagram of an error diffusion processing circuit in a prior art. Here, let us consider an error diffusing system which limits an N-bit input signal to M (<N) bits and outputs M-bit output signal. The main part of the circuit is a quantizing unit 112, where an N-bit input signal is quantized to an M-bit input signal. As this quantizing method, there may be various methods such as a rounding down operation, a rounding up operation, a rounding off operation, an adaptive threshold and the like. Any of them may be used. The quantized result is outputted as the output signal. At the same time, a quantized error signal is also calculated by a subtracter 113. This quantized error signal is spatially diffused in a filter unit 115 and added to the input signal by using an adder 110. If a gain of the filter in the filter unit 115 is 1, it implies that the quantized error induced in a certain pixel is diffused into the peripheral pixels This is the basic configuration of the error diffusing method. Here, by adding the quantized error signal to the input signal, there is a possibility of the overflow/underflow from a dynamic range. Thus, in many cases, a limiter 111 is placed in an actual circuit.
FIGS. 2A and 2B show an example when a signal bit width of an error diffusion feedback loop is limited in the circuit in FIG. 1. In FIG. 2A, the gray tone values of all pixels are set to 3/1024. FIG. 2B shows a one-bit output image generated by using an error diffusion filter of a Floyd-Steinberg type. In this output image, the pixels having gray tone values of 0/256 and 1/256 are properly mixed to thereby attain the average gray tone representation of 3/1024.
However, if the signal bit width of the feedback loop in the above-mentioned error diffusing circuit is limited to the 10 bits, a particular pattern may appear in the output image, as shown in FIG. 2B, where the pixel of 0/255 is represented by black, and the pixel of 1/255 is represented by white for easy understanding. The technique is desired to avoid the particular pattern (as shown in FIG. 2B) appearing in a certain kind of an image, as mentioned above.
Also, when many gray tone representations are tried under the limited number of gray tones, a random dither method, a dither pattern method, an error diffusing method and the like are well known methods. Many methods obtained by combining above-mentioned methods are invented.
For example, Image Processing Apparatus is disclosed in Japanese Laid Open Patent Application (JP-A 2000-287086).
The image processing apparatus processes an input image information from a predetermined image input unit and then outputs it. The image processing apparatus includes a many-valued error diffusing processor, a random noise generator and a noise superimposing processor. The many-valued error diffusing processor has a quantizing processor, a quantized error detector, a peripheral error calculator and an error-superimposing unit. The quantizing processor quantizes sequentially inputted pixel data constituting the image information in accordance with output values distributed at a predetermined gray tone difference. The quantized error detector detects a quantized error of the pixel data. The peripheral error calculator integrates the quantized error of the pixel data detected by the quantized error detector with a quantized error of its peripheral pixel data. The error-superimposing unit for feedback adding an integration error calculated by the peripheral error calculator to a next input pixel data. The random noise generator generates a random noise, on the basis of a gray tone level of an input pixel data. The noise superimposing processor superimposes, on the pixel data, the random noise generated by the random noise generator before the many-valued error diffusing processor performs the many-valued error diffusing process on the pixel data.
This provides an image processing apparatus that can suppress a pseudo gray tone phenomenon through a many-valued error diffusing process.
In the image processing apparatus, an operation for adding random noise is added to an error diffusing process in order to increase an operational precision of an error diffusion loop circuit. However, the operation for adding the noise is not added to the error diffusion loop, and it is added before the loop process. Thus, a natural error diffusion pattern can not be obtained.
Also, in Image Processing Method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei, 9-247451).
The image processing method carries out a two-valued process for converting a many-valued image into a two-valued image through an error diffusion and an encoding process for encoding the two-valued image obtained by the two-valued process. It provides a relative relation between a range of the error diffusion in the two-valued process and a process data unit treated by the encoding process.
The image processing method aims to provide an image processing method that can improve a compressing efficiency, when an encoding process compresses a pseudo halftone image obtained by an error diffusing method, from a many-valued image.
In the image processing method, a random number generator is added to an error diffusing circuit. However, even in this method, the operation for adding the noise is not added to an error diffusion loop.
Also, Pseudo Gradating Circuit And Pseudo Gradating Method is disclosed in Japanese Laid Open Patent Application (JP-A 2001-218052).
The pseudo-gradating circuit includes a memory, an adder, an adder and a controller The memory stores a predetermined data series. The adder adds the predetermined data series to an image data. The adder adds an inputted input image data and an error data in a pseudo-gradated image data. The controller carries out a control to add 0 without adding the data series and the error data in accordance with a predetermined control signal. Here, a dithering process and an error diffusing process are executed by one circuit.
The pseudo-gradating circuit aims to make a size of a circuit smaller and make a cost lower while enabling a dithering process and an error diffusing process to be carried out by one substrate. . It also aim to protect a dot in a white background from being generated even if a discontinuity of a gray tone is corrected by the error diffusing process. It also aim to protect a color deviation in a colored character and a uniform image region from being generated even if an error data is added by the error diffusing process.
In the pseudo-gradating circuit, a noise signal is added to an error diffusing circuit so that an error diffusing method and a dithering method are switched by one circuit. However, this does not improve the error diffusing method.
Moreover, Image Processing Apparatus is disclosed in Japanese Laid Open Patent Application (JP-A 2001-119579).
The image processing apparatus performs an N-valued (M>N) process on an M-valued image data of a remarkable pixel in accordance with an error diffusing method The image processing apparatus includes a correcting unit, a calculating unit and an output unit. The correcting unit corrects the M-valued image data of the remarkable pixel on the basis of an N-valued error of its peripheral pixel and uses it as a correction image data. The calculating unit compares the correction image data with a threshold and calculates an N-valued image data of the remarkable pixel. The output unit outputs an N-valued error having the number of bits less than that of the correction image data by using a many-valued dithering method, in accordance with the correction image data and the N-valued image data.
The image processing apparatus provides an image processing apparatus using an error diffusing method that can carry out a high-speed process.
In the image processing apparatus, a dither operation is performed on a signal before a quantizing operation, among two signals to obtain a quantized error signal. However, the signal itself before the quantizing operation is only dithered. A signal such as a dither pattern is not added as noise.
Also, in Image Processing Method, Apparatus for the Same, And Recording Medium is disclosed in Japanese Laid Open Patent Application (JP-A 2001-257880).
The image processing method is for performing a halftone process on an input image by using an error diffusing method. The image processing method includes four steps. The first step is for generating a sine wave on which amplitude and frequency modulations are performed, in accordance with an average value between a remarkable pixel value and a peripheral pixel value. The second step is for adding the diffused quantized error, the remarkable pixel value and the sine wave. The third step is for quantizing the addition result on the basis of a predetermined number of gray tones. The fourth step is for calculating the quantized error of the peripheral pixel from the error caused by the quantizing operation.
The image processing method improves a geometrical texture of an output image and a particle property, in a halftone process that uses an error diffusing method.
In the image processing method, the circuit is designed such that a sine wave is added to an adder 10. However, in this technique, its circuit configuration is complex since a sine wave on which amplitude and frequency modulations are performed in accordance with an average value between remarkable pixel and peripheral pixel values is added.